1. Field of the Invention
The invention relates to a plasma display panel, and more particularly to an AC type plasma display panel.
2. Description of the Related Art
A plasma display panel is structurally grouped into a DC type panel in which electrodes are exposed to discharge gas and an AC type panel in which electrodes are covered with a dielectric layer, and accordingly, are not exposed to discharge gas. An AC type plasma display panel is further grouped into a memory operation type panel having a memory function caused by electric-charge accumulation function of the above-mentioned dielectric layer, and a refresh operation type panel not having such a memory function.
FIG. 1 is an exploded perspective view of a conventional plasma display panel. Hereinbelow are explained a structure of an AC type plasma display panel and a method of driving a memory function type plasma display panel.
The plasma display panel is designed to include an electrically insulating front substrate 1A and an electrically insulating rear substrate 1B.
A scanning electrode 9 and a sustaining electrode 10, spaced away from each other by a predetermined gap, are arranged on the front substrate 1A as a pair in parallel with each other. Each of the scanning and sustaining electrodes 9 and 10 is comprised of a principal electrode 2 for generating discharge and a bus electrode 3 for ensuring electrical conductivity.
The principal electrode 2 is comprised of a transparent electrode composed of ITO (indium tin oxide) or SnO2 in order to prevent reduction in a light-transmission rate. The bus electrode 3 is composed of metal such as silver.
The scanning and sustaining electrodes 9 and 10 are covered with a dielectric layer 4A, and the dielectric layer 4A is covered with a protection film 5 composed of magnesium oxide for protecting the dielectric layer 4A from discharge.
A plurality of data electrodes 6 is arranged on the rear substrate 1B such that the data electrodes 6 extend in a direction perpendicular to a direction in which the scanning and sustaining electrodes 9 and 10 extend.
The data electrodes 6 are covered with a dielectric layer 4B. On the dielectric layer 4B is formed a plurality of partition walls 7 each extending in a column direction and defining discharge spaces and cells.
Phosphor 8 is coated on an exposed surface of the dielectric layer 4B and sidewalls of the partition walls 7. The phosphor 8 converts ultra-violet rays generated by discharge, into a visible light. By coating red, green and blue phosphors in every three cells, it would be possible to display images with colors.
The front and rear substrates 1A and 1B are adhered to each other in hermetically sealed condition such that the protection film 5 and the phosphor 8 face each other. Discharge gas composed of helium, neon or xenon solely or in combination is introduced into spaces sandwiched defined by the front and rear substrates 1a and 1b and the partition walls 7.
FIG. 2A is a plan view of the plasma display panel illustrated in FIG. 1, as viewed from a viewer.
The scanning and sustaining electrodes 9 and 10 extend in a row direction in parallel with each other as a pair. A gap formed between the scanning and sustaining electrodes 9 and 10 defines a discharge gap, in which surface discharge is generated between the scanning and sustaining electrodes 9 and 10.
Hereinbelow is explained generation of discharge in a selected display cell.
By applying a pulse voltage across the scanning and data electrodes 9 and 6 in each of display cells over a discharge threshold, discharge is generated between the scanning and data electrodes 9 and 6. As a result, positive and negative electric charges are attracted to and accumulated on surfaces of the dielectric layers 4A and 4B in accordance with polarity of the pulse voltage.
A wall voltage defined as an equivalent internal voltage caused by accumulation of the electric charges has opposite polarity to a polarity of the pulse voltage. Hence, as the discharge grows, an effective voltage in a display cell lowers, resulting in that even if the pulse voltage is kept at a fixed voltage, the discharge cannot be maintained, and finally, stops.
If a voltage equal to or greater than a predetermined voltage is applied across the scanning and sustaining electrodes 9 and 10 when discharge is generated between the scanning and data electrodes 9 and 6, there is further generated discharge between the scanning and sustaining electrodes 9 and 10 with the former discharge acting as a trigger. As a result, similarly to the discharge generated between the scanning and data electrodes 9 and 6, electric charges are accumulated on the dielectric layer 4A such that the voltage applied is cancelled.
Then, a sustaining discharge pulse defined as a pulse voltage having the same polarity as that of the wall voltage is applied across the scanning and sustaining electrodes 9 and 10. Since the wall voltage is added as an effective voltage to the sustaining discharge pulse, even if the sustaining discharge pulse has a small voltage-amplitude, a sum of the wall voltage and the sustaining discharge pulse is over a discharge threshold, and hence, there is generated discharge. Accordingly, the discharge can be maintained by alternately applying the sustaining discharge pulse across the scanning and sustaining electrodes 9 and 10. This is called a memory function.
FIG. 3 illustrates waveforms of voltages to be applied to electrodes in a conventional method of driving a plasma display panel. Hereinbelow is explained a method of driving a memory function and AC type plasma display panel, with reference to FIG. 3.
In FIG. 3, “Si” indicates a waveform of a voltage to be applied to scanning electrode 9 scanned at an i-th order, “C” indicates a waveform of a voltage to be applied to the sustaining electrode 10, and “D” indicates a waveform of a voltage to be applied to the data electrode 6.
As illustrated in FIG. 3, one period of driving a plasma display panel is comprised of an initialization period in which a display cell is initialized for readily generating discharge, a scanning period in which a cell or cells from which a light is emitted is(are) selected, and a sustaining period in which a light is emitted in a cell or cells having been selected in the scanning period.
In the initialization period, an erasion pulse P1 is applied to all of the scanning electrode 9 for generating erasion discharge for erasing wall electric charges accumulated on the dielectric layers 4A and 4B by the previous sustaining discharge pulses.
Herein, the term “erasion” or “erasing” is not to be limited to erasion of all of wall electric charges, but should be interpreted to include adjustment of wall electric charges for smoothly carrying out subsequent preliminary discharge, writing discharge and sustaining discharge.
Then, a preliminary discharge pulse P2 is applied to all of the scanning electrodes 9 for compulsorily generating discharge in all of display cells for light emission. Then, a preliminary discharge erasing pulse P3 is applied to all of the scanning electrode 9 for generating erasing discharge to erase wall electric charges generated by the preliminary discharge pulse P2.
The term “erase” is not to be limited to erasion of all of wall electric charges, but should be interpreted to include adjustment of wall electric charges for smoothly carrying out subsequent writing discharge and sustaining discharge. Subsequent writing discharge can be readily generated by virtue of the preliminary discharge and the preliminary discharge erasion.
The preliminary discharge pulse P2 and the preliminary discharge erasing pulse P3 both illustrated in FIG. 3 are serrate pulses in which a voltage is increased with the lapse of time. The serrate pulse generates weak discharge expanding only around the discharge gap.
The preliminary discharge and the discharge for erasing the preliminary discharge are generated independently of images. Hence, light emission caused by these discharges is observed as a background luminance. If such a background luminance is high, contrast would be deteriorated, and image quality is degraded.
In the scanning period, a scanning pulse P4 is applied to each one of the scanning electrodes 9 at different timings, and a data pulse P5 is applied to the data electrodes 6 in accordance with displayed data at a timing at which the scanning pulse P4 was applied to the scanning electrode 9.
In a cell in which the data pulse P5 is applied to the data electrode 6 when the scanning pulse P4 was applied to the scanning electrode 9, discharge is generated between the scanning and data electrodes 9 and 6, and then, the discharge triggers another discharge between the scanning and sustaining electrodes 9 and 10.
A series of the above-mentioned steps is called writing discharge. As a result of generation of the writing discharge, positive electric charges are accumulated on the dielectric layer 4A above the scanning electrodes 9, negative electric charges are accumulated on the dielectric layer 4A above the sustaining electrodes 10, and negative electric charges are accumulated on the dielectric layer 4B above the data electrodes 6.
In the sustaining period, there is generated surface discharge between the scanning and sustaining electrodes 9 and 10, if a voltage caused by electric charges accumulated on the dielectric layer 4a due to the writing discharge having been generated in the scanning period is added to a sustaining voltage.
The sustaining voltage is designed to be a voltage smaller than a voltage at which the surface discharge is generated. Hence, if the writing discharge was not generated in the scanning period, and hence, wall electric charges were not accumulated on the dielectric layer 4A, sustaining discharge is generated for displaying images only in a cell having been selected in the scanning period.
After generation of the first sustaining discharge, negative electric charges are accumulated on the dielectric layer 4A above the scanning electrodes 9, and positive electric charges are accumulated on the dielectric layer 4A above the sustaining electrodes 10.
The second sustaining pulse has a polarity opposite to that of the first sustaining pulse to be applied to the scanning and sustaining electrodes 9 and 10. Hence, a voltage caused by electric charges accumulated on the dielectric layer 4A is added to the second sustaining pulse, and resultingly, there is generated second discharge. Thereafter, sustaining discharge is kept generated in the same way.
If the surface discharge is not generated by the first sustaining pulse, there is not generated discharge by subsequent sustaining pulses.
A combination of the above-mentioned initialization period, scanning period and sustaining period is called a sub-field. Images are displayed by turning on or off each of a plurality of sub-fields.
In the above-mentioned method of driving a plasma display panel, a luminance of a plasma display panel is defined as a product of a light-emission luminance per a sustaining pulse and the number of sustaining light emission, and consumed power is defined as a product of a voltage of a sustaining pulse and a current.
However, the conventional plasma display panel is accompanied with high power consumption. For instance, a 42-size plasma display panel consumes about 150 to 200 W for generating sustaining discharges. Thus, it is desired that a current is reduced for reducing power consumption.
FIG. 2A illustrates a shape of the principal discharge electrode 2, and FIG. 2B illustrates an area 17 in which weak discharge expands.
If the principal discharge electrode 2 has such a shape as illustrated in FIG. 2A, a current is consumed much relative to a light-emission luminance, resulting in a problem of a poor light-emission efficiency.
If discharge is generated in the vicinity of the partition walls 7, electric charges generated in the discharge are likely to be attracted to the partition walls 7, resulting in reduction in ultra-violet rays. That is, a current is not effectively consumed with the result of a problem of deterioration in a light-emission efficiency.
FIG. 4A is an upper plan view of another principal discharge electrode 2, and FIG. 4B shows an area 16 in which strong discharge expands.
The illustrated principal discharge electrode 2 is designed to be rectangular and exist only in a display area, unlike the principal discharge electrode 2 illustrated in FIG. 1 which extends in the row direction. The rectangular principal discharge electrode 2 illustrated in FIG. 4A prevents generation of discharge in the vicinity of the partition walls 7, and further prevents electric charges from being attracted to the partition walls 7. Hence, the rectangular principal discharge electrode 2 illustrated in FIG. 4A presents a slightly higher light-emission efficiency than that of the principal discharge electrode 2 illustrated in FIG. 2A.
However, a light-emission efficiency presented by the rectangular principal discharge electrode 2 illustrated in FIG. 4A is not sufficient. This is because the principal discharge electrode 2 covers a display area almost entirely therewith, and hence, discharge is generated over entirety of the principal discharge electrode 2 and hence entirety of a display area.
Ultra-violet rays are also generated in an area in which discharge is generated. If some time has passed after generation of ultra-violet rays, ultra-violet rays are absorbed into surrounding discharge gas, resulting in that ultra-violet rays cannot reach the phosphor 8. Accordingly, ultra-violet rays are not converted into a visible light, unless the ultra-violet rays are generated in the vicinity of the partition walls 7 on which the phosphor 8 is coated.
The principal discharge electrodes 2 over which discharge and ultra-violet rays are generated, illustrated in FIGS. 2A and 4A, are accompanied with a problem that much ultra-violet rays are not converted into a visible light with the result of reduction in a light-emission efficiency.
FIG. 5 is a plan view of a principal discharge electrode suggested in Japanese Patent Application Publication No. 2001-160361.
As illustrated in FIG. 5, the suggested principal discharge electrode 2 is comprised of a first linear portion defining a discharge gap, and two second linear portions extending from opposite ends of the first linear portion along a partition wall 7. The suggested principal discharge electrode 2 is accompanied with a problem that it is difficult for strong discharge generated at the center of the first linear portion to expand towards distal ends of the second linear portions. As a result, a plasma display panel cannot operate stably and a luminance is low in comparison with the principal discharge electrode illustrated in FIG. 4A.
FIG. 6 is a plan view of another principal discharge electrode suggested in Japanese Patent Application Publication No. 2001-160361.
As illustrated in FIG. 6, the suggested principal discharge electrode 2 is comprised of a first linear portion defining a discharge gap, and two second linear portions radially extending from opposite ends of the first linear portion. The first linear portion is designed shorter than the first linear portion illustrated in FIG. 5. This results in that a voltage at which discharge is generated becomes higher than the same in FIG. 5. In addition, since strong discharge generated at the center of the first linear portion does not expand to the partition wall 7 along the discharge gap, a luminance is unavoidably reduced.
Japanese Patent Application Publication No. 10-321142 has suggested a plasma display panel including sustaining electrodes each comprised of a transparent electrode having a high rate at which a visible light passes therethrough, and a mother electrode composed of a low-resistive metal and extend in parallel with the transparent electrode. The transparent electrode and the mother electrode are electrically connected to each other through a short-circuit electrode which overlaps a rib extending in a direction perpendicular to a direction in which the sustaining electrodes extend.
Japanese Patent Application Publication No. 2000-323045 has suggested a plasma display panel including principal electrodes each having a bus in the form of a strip and extending in a row direction, and gap-forming portions extending from the bus in a column direction.
Japanese Patent Application Publication No. 2001-307646 has suggested a gas discharge panel including a pair of display electrodes each comprised of a bus line extending in a row direction, and a plurality of extensions extending towards an opposing display electrode. The extensions extending from the display electrodes are alternately arranged.
Japanese Patent Application Publication No. 2002-8549 has suggested a plasma display panel including sustaining electrodes each formed with a plurality of openings arranged in a matrix. The openings are in the form of a rectangle having a side smaller than 30 micrometers.
Japanese Patent Application Publication No. 2002-75219 has suggested a plasma display panel including a bus electrode having a portion extending towards two light-emission areas located adjacent to each other such that the portion is electrically connected to a transparent electrode.